In recent years, demands for reduction in electric power consumption, reduction in a thickness and flexibility of display devices are increased as information devices develop, and research and development of display devices which meet the above demands are actively carried out.
An electrophoretic display device invented by Harold D. Lees et al. is known as one of such display devices. This electrophoretic display device has a structure in which two electrode substrates at least one of which is transparent are oppositely disposed via suitable spacers and in which a display liquid prepared by dispersing fine particles (pigment particles) in a dispersion medium that is colored differently from the color of the particles is filled into the space between the electrode substrates to form a display panel, and display is shown on the transparent electrode surface by applying an electric field to the display panel (refer to, for example, patent document 1).
A liquid for electrophoretic display filled into the space between the electrode substrates is constituted from fine particles of titanium oxide and the like (pigment particles), a dispersion medium having a low dielectric constant such as xylene, tetrachloroethylene, paraffin and silicone oil in which a dye for giving a contrast of a color to the fine particles is dissolved, a dispersant such as a surfactant and the like and additives such as a charge-providing agent and the like. The fine particles in the display liquid transfer to a transparent electrode side by applying an electric field to the liquid for electrophoretic display to allow the color of the fine particles to appear on the display face. Further, the fine particles transfer to an opposite side by applying an electric field of a direction which is reverse to the above to allow the color of the dispersion medium colored by the dye to appear on the display face.
Such an electrophoretic display device is a display device in which desired display can be obtained by controlling a direction of an electric field, and it has the advantages that it costs low and has as a broad visibility angle as those of printed matters and that it is small in electric power consumption and has a memory property of display, so that it is paid attentions as an inexpensive display device.
However, since a display liquid for the above electrophoretic display device described in the patent document 1 is prepared usually by dispersing an inorganic pigment having a high refractive index such as titanium dioxide and the like in a hydrophobic dispersion medium having a low dielectric constant which is colored by dissolving a dye, it is difficult to control dispersion stability, and it has the defects that coagulation brings about reduction in the contrast and the repetitive display characteristics and reduction in the display quality. Further, since the dye solution is used, it has the defect that the contrast is weakened by adsorption of the dye onto the pigment surfaces.
Accordingly, a lot of developments for stabilizing the dispersion systems and improving the display characteristics has so far been made in order to solve the above problems.
For example, an art in which sorbitan fatty acid ester surfactants are added in a system using electrophoretic particles subjected to surface treatment with a coupling agent of titanate in a dispersion medium colored by a dye is known as a means for raising the dispersion stability (refer to, for example, patent document 2).
Further, it is known that one of two kinds of electrophoretic particles having different charging states is treated with a quaternary ammonium salt compound and a surfactant is further added thereto (refer to, for example, patent document 3).
On the other hand, it is known as a means for improving the contrast that a dye which is non-adsorptive to a pigment surface is used as a dye used for coloring a dispersion medium (refer to, for example, non-patent document 1) and that a dye concentration in a dispersion medium is to be low (refer to, for example, non-patent document 2).
Further, a method in which microcapsules encapsulated a display liquid for electrophoretic display are used as display particles (refer to, for example, patent document 4) is known as a method for preventing unequal display caused by uneven distribution of electrophoretic fine particles on a display face.
However, the art described in the patent document 2 is unsatisfactory in an effect of dispersion stabilization and still has a problem in that a satisfactory effect is not obtained as well in a contrast of display.
Also, a satisfactory dispersion stability is not obtained as well in the art described in the above patent document 3, and the effect is unsatisfactory.
Further, the methods described in the non-patent documents 1 and 2 do not come to completely solve the defects brought about by the dye solutions and still have problems in terms of practical use.
In the method of the patent document 4 described above, a colored dye solution and a dispersion liquid of pigment particles are used for an encapsulated display liquid, and therefore it is not satisfactory in terms of a contrast as is the case with the phenomenon described above.
Then, an electrophoretic display device in which a dye solution is not used is known as a means for solving the problems involved in the systems in which a dispersion medium colored by dyes is used. Known is, for example, an electrophoretic display element in which a liquid prepared by dispersing at least two kinds of electrophoretic fine particles which are different from each other in a color tone and an electrophoretic property in a colorless dispersion medium is filled in a cell formed between two counter electrodes at least one of which is transparent via spacers (refer to, for example, patent document 5).
Also, use of a steric repulsion effect brought about by addition of a charge-controlling agent and surface treatment of particles is known as a means for preventing coagulation between particles which is a problem in a system using, as a display liquid for electrophoretic display, the above liquid prepared by dispersing two kinds of the electrophoretic fine particles which are different from each other in a color tone and an electrophoretic property (electrified charge) (refer to, for example, patent document 6).
Further, known is an electrophoretic display element using a liquid prepared by dispersing at least two kinds of electrophoretic fine particles which have the same electrophoretic property and which are different from each other in an electrophoretic speed in a colorless dispersion medium (refer to, for example, patent document 7).
Also, known is a display liquid for electrophoretic device containing particles which are subjected to surface treatment and have voids in the inside and pigment particles having a color tone which is different from that of the above particles in a dispersion medium (refer to, for example, patent document 8).
Further, known as well is an art in which a liquid prepared by dispersing at least two kinds of electrophoretic fine particles which are different from each other in a color tone and an electrophoretic property in a colorless dispersion medium is encapsulated in microcapsules (refer to, for example, patent document 9).
However, in the art described in the patent document 5 described above, an electrified charge of the different electrophoretic fine particles is combination of a positive charge and a negative charge, and therefore involved therein is the problem that electric attracting force is produced between the fine particles to cause coagulation of the fine particles, whereby the dispersion stability is reduced to bring about deterioration in the contrast by mixed color.
Also, in the art described in the patent document 6 described above, involved therein is the problem that it is difficult to completely prevent two kinds of the electrophoretic fine particles from being coagulated, so that the good contrast can not be realized.
Further, even the fine particles having a slow moving speed described in the above patent document 7 are in a shorter moving distance when present close to the electrode on the display face and therefore appear on the display face in a certain case. Also, an electrified charge is usually different even between particles having the same color tone, and the moving speeds have distribution. Accordingly, a problem is involved in that it is difficult to obtain the satisfactory contrast without controlling so that the particles are not superposed among those having different color tones.
Further, in the particles having voids in the inside described in the above patent document 8, the dispersion liquid medium gets into the voids, and therefore particularly the white particles are reduced in a refractive index, which makes it difficult to obtain the satisfactory contrast.
Also in the case described in the above patent document 9, stability of the dispersion liquid is not maintained, and there still remains the problem that color mixing caused by coagulation due to electric attracting force between the electrophoretic fine particles is generated in the microcapsule to bring about color mixing of display.
Patent document 1: U.S. Pat. No. 3,612,758 (claims, examples and others)
Patent document 2: U.S. Pat. No. 2,733,687 (claims, examples and others)
Patent document 3: Japanese Patent Application Laid-Open No. 119704/1999 (claims, examples and others)
Non-patent document 1: Philips Lab: Conference Record of 1980 Biennial Disp. Res. Conf.
Non-patent document 2: Xerox Palo Alto: Proc. SID, Vol. 18 3/4, 1977
Patent document 4: Japanese Patent No. 2551783 (claims, examples and others)
Patent document 5: Japanese Patent Application Laid-Open No. 269124/1987 (claims, examples and others)
Patent document 6: Published Japanese translation of PCT International Publication for Patent Application No. 510790/1996 (claims, examples and others)
Patent document 7: Japanese Patent Application Laid-Open No. 50886/1988 (claims, examples and others)
Patent document 8: U.S. Patent Application Japanese Patent Application Laid-Open No. 2002-277903 (claims, examples and others)
Patent document 9: U.S. WO98/03896 (claims, examples and others)